Glasgow University is giving theology students what the Daily Mail calls “trigger warnings” about potentially upsetting images of the crucifixion. The theology department concedes that its course about Christ in cinema “contains graphic scenes of the crucifixion, and this is flagged up to students beforehand”. Given it includes Mel Gibson’s blood-spattered The Passion of the Christ, you can understand the anxiety (though the university tells us no students opted out); more recently, Martin Scorsese has featured underwater crucifixions in his new film Silence. Yet long before Jesus was dying on screen he was being nailed up in paintings and in sculpture.

One work that haunts me, and not in a good way, is a painting by Lucas Cranach the Elder that I chanced on a few years ago in the great Alte Pinakothek museum in Munich. Painted in 1503, it shows not only Christ but the two thieves who died beside him. At first sight it may not seem so sensational – except for a nasty spurt of blood out of Christ’s nailed feet. Then you notice that a single huge nail goes through both feet. Oh wait, this really is horrible. For the feet are bizarrely twisted together and shapeless, as if the nail has smashed bones and torn tendons in its violent riveting of flesh to wood.

Such gross deathly distortions can be seen, too, in the roughly slung up, lumpen bodies of the criminals, the slump of a head sagging into a chest, legs hanging as flaccidly as animal skins. Then you see more blood, seeping from the spear wound in Christ’s torso, dripping over the face and throat of a thief who appears to have been beaten and perhaps garrotted before being crucified, streaming down an inert flank.

Cranach – who later served as best man at Martin Luther’s wedding and was a magistrate responsible for several judicial murders of supposed witches – paints the crucifixion with an almost nauseating physical realism. This great and horrible picture reminds me of Francis Bacon in its insistence on the carnal facts of torture and death. Bacon’s own Three Studies for Figures at the Base of a Crucifixion (c1944) is another painting nervous teachers might want to warn their students about. Bizarre gargoyles crouch and gurn, eyeless and many toothed, their grey bodies warped and stumpy. They are, perhaps, creatures made up of the leftover flesh after Christ’s spirit left his body. As in Cranach’s painting, Bacon’s nightmare dwells not on the triumph of the soul but the decay and sickness of our biological selves.

These creatures, too low to be saved, are monsters at the base of the cross, but even Christ himself can look repulsive in art. Mattias Grünewald in 1512-16 gives him grey-green skin covered in sores and scabs. His fingers are twisted and splayed in grotesque positions of paralysed agony. As in Cranach’s version, both feet are pinned together by a single nail, the bones that stick through collapsed skin horribly tautened by this brutality.

Why do Renaissance artists such as Cranach and Grünewald resort to such excess in painting the crucifixion? The theological answer is surely straightforward. God gave his own son to suffer and die as a mortal human. To grasp the nature of this sacrifice you need to picture its full tragic reality. In art, the image of a suffering Christ evolved as medieval Christianity became increasingly personal and introspective. As laypeople took more responsibility for their own piety, and the rich lavished their money on personalised Books of Hours, depictions of the crucifixion became more realistic and harrowing. Late medieval artists dwell not only on the crucifixion but the grief of Christ’s followers as they collect his corpse, the agony of his mourning mother and even the inertness of his lifeless body.

Leaving aside moral panics over “trigger warnings”, “politically correct universities” and a supposed “snowflake generation”, it is interesting that the 21st century may shy away from the graphic nature of Christian images 500 years ago. For we are much less used to death in general. Every old parish church should have a trigger warning, and not only for images of the crucifixion. In any English church with tombs that go back to pre-Victorian times, you are guaranteed to see horribly realistic skulls with empty black eye sockets and grinning, vacant mouths. These grisly icons of death are found on Tudor and Jacobean tombs and on 18th-century memorials

Jesus's tomb in Jerusalem exposed during conservation work

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One reason people before modern times wanted their crucifixions gory and their churches full of images of death was that mortality and its horrors haunted their real lives. Death was everywhere, from the sick beds of people struck down by all the diseases medicine had yet to conquer to public executions whose victims were left to rot on gibbets or, as Bruegel paints them, on open platforms at the tops of wooden poles.

In other words, when artists 500 years ago depicted the crucifixion they were not showing a totally unfamiliar sight. People were still executed and left to rot in public, just as they had been in ancient Roman times. Death was ever present.

It still is, of course, in cells and war zones. But we in wealthy peaceful countries don’t usually see death on the street. We can turn our eyes away more easily from suffering. That is why we need art’s tormenting images of the crucifixion – to make us see what we would rather ignore.